Method and System for Generating Antenna Selection Signals in Wireless Networks

ABSTRACT

Embodiments of the invention describe a method for antenna selection (AS) in a wireless communication network, the network comprising user equipment (UE), wherein the UE comprises a plurality of subsets of antennas including a first subset of antennas and a second subset of antennas, and wherein the UE is configured to transmit a sounding reference signal (SRS) from a subset of antennas at a time. The method transmits a first SRS from the first subset of antennas, transmits a second SRS from the second subset of antennas, receives, in response to the transmitting the first SRS and the second SRS, information identifying an optimal subset of antennas from the first subset of antennas and the second subset of antennas, and transmits user data from the optimal subset of antennas.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/953,452, “Method and System for Generating Antenna Selection Signalsin Wireless Networks,” filed on Dec. 10, 2007, by Mehta et al.

FIELD OF THE INVENTION

This invention relates generally to generating antenna selection signalsin wireless communication networks, and more particularly to selectingantennas in transceivers where the number of RF chains is less than thenumber of antennas.

BACKGROUND OF THE INVENTION OFDM

In a wireless communication network, such as the 3^(rd) generation (3G)wireless cellular communication standard and the 3GPP long termevolution (LTE) standard, it is desired to concurrently support multipleservices and multiple data rates for multiple users in a fixed bandwidthchannel. One scheme adaptively modulates and codes symbols beforetransmission based on current channel estimates. Another optionavailable in LTE, which uses orthogonal frequency division multiplexedaccess (OFDMA), is to exploit multi-user frequency diversity byassigning different sub-carriers or groups of sub-carriers to differentusers or UEs (user equipment). The system bandwidth can vary, forexample, from 1.25 MHz to 20 MHz. The system bandwidth is partitionedinto a number of subcarriers, e.g., 1024 subcarriers for a 5 MHzbandwidth.

The following standardization documents are incorporated herein byreference: 36.211, 3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network; Physical Channels andModulation (Release 8), v 1.0.0 (2007-03); R1-01057, “Adaptive antennaswitching for radio resource allocation in the EUTRA uplink,” MitsubishiElectric/Nortel/NTT DoCoMo, 3GPP RAN1#48, St. Louis, USA; R1-071119, “Anew DM-RS transmission scheme for antenna selection in E-UTRA uplink,”LGE, 3GPP RAN1#48, St. Louis, USA; and “Comparison of closed-loopantenna selection with open-loop transmit diversity (antenna switchingwithin a transmit time interval (TTI)),” Mitsubishi Electric, 3GPPRAN1#47bis, Sorrento, Italy. According to the 3GPP standard, the basestation is enhanced, and is called the “Evolved NodeB” (eNodeB).

MIMO

In order to further increase the capacity of a wireless communicationsystem in fading channel environments, multiple-input-multiple-output(MIMO) antenna technology can be used to increase the capacity of thesystem without an increase in bandwidth. Because the channels fordifferent antennas can be quite different, MIMO increases robustness tofading and also enables multiple data streams to be transmittedconcurrently.

While MIMO systems perform well, they also can increase the hardware andsignal processing complexity, power consumption, and component size intransceivers. This is due in part to the fact that each receive antennarequires a receive radio frequency (RF) chain, which typically comprisesa low noise amplifier, a frequency down-converter, and an analog todigital converter. Similarly, each transmit antenna element requires anRF chain that comprises a digital to analog converter, a frequencyup-converter, and a power amplifier.

Moreover, processing the signals received in spatial multiplexingschemes or with space-time trellis codes requires receivers where thecomplexity can increase exponentially as a function of the number ofantenna.

Antenna Selection

Antennas are relatively simple and cheap, while RF chains areconsiderably more complex and expensive. Antenna selection reduces someof the complexity drawbacks associated with MIMO systems. Antennaselection reduces the hardware complexity of transmitters and receiversby using fewer RF chains than the number of antennas.

In antenna selection, a subset of the set of available antennas isadaptively selected by a switch, and only signals for the selectedsubset of antennas are connected to the available RF chains for signalprocessing, which can be either transmitting or receiving. As usedherein, the selected subset, in all cases, means one or more of all theavailable antennas in the set of antennas. Note, that invention alsoallows multiple subsets to be used for training. For example, there canbe four antennas and one RF chain, or eight antennas and two RF chains,which includes four subsets.

Antenna Selection Signals

Pilot Tones or Reference Signals

In order to select the optimal subset of antennas, all channelscorresponding to all possible transmit and receive antenna subsets needto be estimated, even though only a selected optimal subset of theantennas is eventually used for transmission.

This can be achieved by transmitting antenna selections signals, e.g.,pilot tones, also called reference signals, from different antennas orantenna subsets. The different antenna subsets can transmit either thesame pilot tones or use different ones. Let N_(t) denote the number oftransmit antennas, N_(r) the number of receive antennas, and letR_(t)=N_(t)/L_(t) and R_(r)=N_(r)/L_(r) be integers. Then, the availabletransmit (receive) antenna elements can be partitioned into R_(t)(R_(r))disjoint subsets. The pilot repetition approach repeats, for R_(t)×R_(r)times, a training sequence that is suitable for an L_(t)×L_(r) MIMOsystem. During each repetition of the training sequence, the transmit RFchains are connected to different subsets of antennas. Thus, at the endof the R_(t)×R_(r) repetitions, the receiver has a complete estimate ofall the channels from the various transmit antennas to the variousreceive antennas.

In case of transmit antenna selection in frequency division duplex (FDD)systems, in which the forward and reverse links (channels) are notidentical, the receiver feeds back the optimal set of the selectedsubset of antennas to the transmitter. In reciprocal time divisionduplex (TDD) systems, the transmitter can perform the selectionindependently.

For indoor LAN applications with slowly varying channels, antennaselection can be performed using a media access (MAC) layer protocol,see IEEE 802.11n wireless LAN draft specification, I. P802.11n/D1.0,“Draft amendment to Wireless LAN media access control (MAC) and physicallayer (PHY) specifications: Enhancements for higher throughput,” Tech.Rep., March 2006.

Instead of extending the physical (PHY) layer preamble to include theextra training fields (repetitions) for the additional antenna elements,antenna selection training is done by the MAC layer by issuing commandsto the physical layer to transmit and receive packets by differentantenna subsets. The training information, which is a single standardtraining sequence for a L_(t)×L_(r) MIMO system, is embedded in the MACheader field.

OFDMA Structure in LTE

The basic uplink transmission scheme is described in 3GPP TR 25.814,v1.2.2 “Physical Layer Aspects for Evolved UTRA.” The scheme is asingle-carrier transmission (SC-OFDMA) with cyclic prefix (CP) toachieve uplink inter-user orthogonality and to enable efficientfrequency-domain equalization at the receiver side.

LTE Reference Signals

3GPP LTE envisages using two kinds of reference signals. Both thereference signals are transmitted in one or more of the long blocks (LB)of the TTI, or its short blocks, if available.

Data Modulation Reference Signals

The data modulation (DM) reference signal is transmitted along with datain the subcarriers assigned to the user equipment. These signals helpthe eNodeB (Base station) receiver to acquire an accurate estimate ofthe channel, and thereby coherently decode the received signal.

Broadband Sounding Reference Signals (SRS)

The broadband SRS is meant to help the eNodeB to estimate the entirefrequency domain response of the uplink channel from the user to theeNodeB. This helps frequency-domain scheduling, in which a subcarrier isassigned, in principle, to the user with the best uplink channel gainfor that subcarrier. Therefore, the broadband SRS can occupy the entiresystem bandwidth, e.g., 5 MHz or 10 MHz. Alternatives have also beenproposed in which the broadband SRS occupies a fraction of the systembandwidth and is frequency hopped over multiple transmissions in orderto cover the entire system bandwidth.

SUMMARY OF THE INVENTION

The objective of the invention is to provide training for antennasselection (AS) without introducing new reference signals.

In a conventional mobile network, a transmitter in the UE transmits asounding reference signal (SRS) so that a base station can estimate thefrequency response of the entire uplink channel from the UE to the basestation.

If the UE has a single radio frequency (RF) chain and multiple antennas,then an optimal antenna needs to be selected. Antennas are typicallyselected using pilot tones. Thus, in the conventional network, thetransmitter needs to send both the SRS for estimating the channelresponse, and pilot tones for selecting antennas. Thus, an additionalsignals would need to be defined for enabling antenna selection, whichincreases the complexity of the network, and makes the antenna selectionmethod incompatible with conventional UE.

Embodiments of the invention provide a solution for abovementionedproblem by reusing the SRS, which is designed for estimating thefrequency response on the uplink channel to also enable antennaselection. Thus, channel estimation and antenna selection can beperformed by the same signal, and the network does not need to bemodified.

Embodiments of the invention describe a method for antenna selection(AS) in a wireless communication network, the network comprising userequipment (UE), wherein the UE comprises a plurality of subsets ofantennas including a first subset of antennas and a second subset ofantennas, and wherein the UE is configured to transmit a soundingreference signal (SRS) from a subset of antennas at a time. The methodtransmits a first SRS from the first subset of antennas, transmits asecond SRS from the second subset of antennas, receives, in response tothe transmitting the first SRS and the second SRS, informationidentifying an optimal subset of antennas from the first subset ofantennas and the second subset of antennas, and transmits user data fromthe optimal subset of antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a baseband transmission chain according toan embodiment of the invention;

FIG. 1B-1E are block diagrams of transmit time intervals according toembodiments of the invention;

FIG. 1F is a block diagram of a resource block according to anembodiment of the invention;

FIG. 2-13 are block diagrams of antenna selection signals according toembodiments of the invention;

FIGS. 14 and 15 are block diagrams of antenna selection packetsaccording to an embodiment of the invention;

FIG. 16 is a flow diagram of an antenna selection method according to anembodiment of the invention;

FIG. 17 is a block diagram of four ways that antennas can be switchedafter they have been selected;

FIG. 18 is a block diagram of transmitting a sounding reference signalalternatively by different antennas according to an embodiment of theinvention;

FIG. 19 is a block diagram of transmitting a sounding reference signalfrom unselected antennas less frequently than from selected antennasaccording to an embodiment of the invention; and

FIG. 20 is a block diagram of transmitting a sounding reference signalfrom unselected antennas less frequently even when no data aretransmitted from the selected antenna according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the invention provide a method and system forgenerating and transmitting antenna selection signals selection inwireless networks. More specifically, the invention can be used intransceivers where the number of RF chains is less than the number ofantennas, e.g., one transmit RF chains for two transmit antennas, or twotransmit RF chains for four transmit antennas. It should be understoodthat the receive antennas can also be coupled to correspondingly fewerreceive RF chains. It should be noted that the techniques describedherein can work with receivers that have only a single antenna. Theinvention is applicable to networks designed according to 3GPP, 4Gcellular, WLAN, WiBro, WiMAX, and IEEE 802.20 standards.

FIG. 1A shows a portion of a transmit RF chain 10 for a discrete Fouriertransform (DFT)-spread OFDM transceiver used by the embodiments of theinvention. Symbols 11 are encoded using a DFT 12, followed bysub-carrier mapping 13, and an inverse fast Fourier transform (IFFT) 14,followed by cyclic prefix (CP) insertion 14 to produce the transmittedsignal 15. The sub-carrier mapping 13 determines the frequencies usedfor transmission.

FIG. 1B shows the basic structure of a transmission subframe or timeinterval (TTI) 20. The transmission is divided into time slots ofduration 0.5 ms. A frame is 10 ms long. Hereinafter, the term TTI andsubframe are used interchangeably. A TTI includes one or more timeslots. The TTI 21 includes long blocks (LB) 22 and short blocks (SB)separated by CPs 24. In the case the TTI is 1.0 ms long, the frame has12 LBs and 4 SBs.

FIG. 1C shows a TTI with one time slot considered for 3GPP LTE. The TTIhas a duration of 0.5 ms. The TTI includes cyclic prefixes (CP) 24, longblocks (LB) 22, and short blocks (SB) 23. The symbols in the shortblocks can be used to transmit pilot tones 25. The long blocks are usedto transmit information or control symbols (user data). Thus, the TTIincludes six LBs and two SBs.

FIG. 1C shows another 0.5 ms TTI with one time slot considered for 3GPPLTE. In this case, all the OFDM symbols are of the same length and arelong blocks. One or more of the LBs in the time slot are used totransmit pilot tones, while the other LBs are used to transmitinformation (data). In effect, this TTI consists of seven LBs.

FIG. 1E shows a TTI with multiple time slots 26. For example, the TTI is1.0 ms long and includes two time slots 26. The time slots can bepartitioned as in FIGS. 1B-1D. In this case, the TTI consists of 14 LBs.

FIG. 1F shows the basic structure of a resource block (RB) 21 during thetransmission time interval (TTI) 20 according to an embodiment of theinvention. The vertical axis indicates frequency, and the horizontalaxis time, which is partitioned as per the earlier description of a TTI.Therefore, the RB is partitioned in time into the long blocks (unshaded)22, and the short blocks (shaded) 23, e.g., 6 or 12 long blocks, and 2or 4 short blocks. The long blocks are used for control and datasignals, and the short blocks are used for data modulation (DM)reference signals. Another possible structure of the RB includes onlylong blocks and no short blocks, e.g., 7 LBs in a 0.5 ms slot or 14 LBsin a 1.0 ms duration. In frequency domain, the resource block consistsof a number of subcarriers, e.g., 14 subcarriers.

If SBs are present, they are used for DM and broadband SRSs. If only LBsare present in the RB, then one or more of the LBs is used for the DM RSand broadband SRS. Multiple RBs can be assigned to an uplink user. TheseRBs can but need not be contiguous. Furthermore, the subcarriers thatcomprise an RB can be contiguous or distributed over the systembandwidth or a portion thereof.

The DM and broadband sounding reference signals can also be used for thepurpose of antenna selection training. The DM/broadband RS and AS RS canbe the same, it is only their use that differs. DM signals are used todemodulate the data in the other long blocks, while the AS RSs are usedfor channel estimation for the purpose of antenna selection. The use ofthe broadband SRS for antenna selection has the advantage offacilitating joint frequency domain scheduling and antenna selection. Itshould be noted, that channel estimation in a receiver is well known. Itshould be noted that the invention is not limited to a specific numberof long and short blocks during the TTI. For clarity, the CPs are notshown in FIG. 1F.

The antenna selection signals according to one embodiment of theinvention can use orthogonal frequency division multiplexing (OFDM).

To enable antenna selection for the uplink from user equipment (UE) to abase station (BS), the UE transmits DM RS or broadband SRSs from subsetsof available transmit antennas. The BS estimates the channels andselects an optimal (best) subset of the transmit antennas. In case of anFDD system, the BS also feeds back information related to the selectedsubset of antennas to the UE. Then, the UE uses the selected transmitantennas for future transmissions to the BS. The selecting can beperformed periodically, or on demand. In the later case, notification isrequired before training or selecting can commence. It should be noted,that the selection can be for the same antenna subset that waspreviously used, The selected antennas can also be used for receivinguser data, which is well suited for a slow-varying TDD system in whichthe uplink and downlink channels are reciprocal.

Given that there are fewer RF chains than antennas, the pilot tones aretransmitted by different sets of antennas using frequency divisionmultiplexing (FDM) or code division multiplexing (CDM) in a timedivision multiplexed (TDM) manner, consistent with the basic RBstructure shown in FIG. 1F.

In the description below, we first consider a UE with one RF chain andtwo transmit or receive antennas, and FDM pilot tones. These schemes arethen extended for use with CDM pilot tones. Next, we consider two RFchains and four antennas for both FDM and CDM pilot tones. Furtherextensions based on this description are also possible. We then considerschemes that differentiate between the selected subset of antennas,i.e., the ones that are transmitting or have transmitted data currently,and the other unselected antennas. The differentiation lies in how oftenthe AS RS is transmitted by selected and unselected antenna subsets.

As defined herein, the selected antenna subset most recently transmitteduser data, while the unselected antenna subsets usually only transmitthe antenna selection signals. As per an embodiment of the invention,the unselected antenna subsets transmit the AS signals less frequentlythan the selected antenna subset.

We consider two cases of antenna training: antenna training andselection occur within one TTI, and antenna training and selection occurbetween multiple TTIs.

For each of these cases, we describe periodic and on demand antennaselection. We describe the use of various alternative pilot tones forantenna selection, such as the data modulation (DM) RSs, broadband SRSs,or hybrid schemes.

In the examples below for UE with one transmit RF chain and two antennas(Tx1 and Tx2), we assume that one block (SB1) is used to transmit data,control and DM signals, while the another (SB2) is used to transmitperiodically AS signals for the slot structure with six LBs and two SBs.For the 1 ms TTI that includes LBs (and no SBs), two LBs, e.g., LB4 andLB11, are used to transmit the DM and broadband SRSs. The BS estimatesthe channel from the reference signals, and makes an antenna selectiondecision accordingly. For the purpose of this description, we assumethat there is a delay between BS notification of the selection and theactual switching in the UE.

Training when Antenna Selection Occurs within a TTI

Using DM RSs

As shown in FIGS. 2A and 2B, antenna selection and training can beperformed by generating the AS signal periodically. FIG. 2A showsantenna selection every second TTI, and FIG. 2B shows antenna selectionevery third TTI.

As shown in FIG. 2A, during a first TTI 220, the UE begins bytransmitting most of the RB 221, including all long blocks and the DMsignal 201 in SB1 with a selected antenna, e.g., the antenna Tx1 to beused is known to the BS. However, the AS signal 202 of the RB 221 istransmitted from an unselected antenna (Tx2) in SB2. As used herein,unselected means using another subset of antennas than the subset ofantennas used for transmitting the data symbols in the RB most recently.That is, the UE transmits symbols in one TTI with different subsets ofantennas.

For the 1 ms TTI, which includes 3 slots with 2 LBs used for DM signals,the UE begins by transmitting most of the RB, including all the LBs(1-3, 5-14) for data and LB4 for DM signal. However, the AS signal ofthe RB is transmitted from an unselected antenna in LB11.

As shown, the AS signal 202 can be a “low overhead” signal because iteither uses fewer reference signal carriers, e.g., half the number asshown in FIG. 2A, in the case of FDM signals tones, or a lower power forCDM signals.

The BS selects 210 a subset (one in the case of two antennas) ofantennas using the DM signal 201 for Tx1 and the AS signal 202 for Tx2.Some time after making the selection, the BS feeds back the selection,e.g., “use Tx2 205,” to the UE. The UE switches to the selected transmitantenna Tx2 for the next TTI after receiving the feedback. As shown inFIGS. 2A and 2B, this training process is repeated periodically.

FIGS. 2A-2B also show that the amount of overhead, in terms of returnpath forwarding (RPF) for FDM signals and power for CDM signals, can bereduced because the estimation accuracy required for antenna selectionis less than required for coherent demodulation. The amount of overheadreduction involves a trade-off between selection accuracy and pilot toneoverhead reduction.

Using Broadband SRSs

As shown in FIGS. 3A and 3B for the uplink channel, antenna training andselection can also be implemented using broadband sounding RS 302. Thesesignals are known as channel quality indicator (CQI) pilot signals. TheCQI signals are transmitted to enable channel selection and frequencydomain allocation at the BS. Note, the bandwidth of the CQI pilots isgreater than the bandwidth of the RB.

As shown in FIGS. 3A-3B, one block (SB1) is used for the data signals(long blocks) and the DM signals 301 of most of the RB, and the otherblock (SB2) is used for the CQI signals 302. FIG. 3A shows joint antennaselection and resource block assignment using the CQI signals 302 forevery TTI.

FIG. 3B shows joint antenna selection and resource block assignmentusing the CQI signals transmitted every multiple TTIs, e.g., two ormore. Generally, the CQI signals are transmitted in every TTI, orperiodically every multiple TTIs.

This enables the BS to estimate the broadband frequency response of thechannels for both antennas. Using the CQI signals for training andselection has the additional advantage of enabling joint resource blockcarrier frequency assignment and antenna selection, which improves theefficiency of frequency domain scheduling. The UE can switch transmitantennas as well as frequencies used in the RB.

On-Demand Adaptive Antenna Training and Selection

Instead of transmitting the AS signals periodically, the AS signals canbe transmitted only when the performance of the current antenna fallsbelow a desired threshold as shown in FIG. 4. A history of signalinterference and noise ratio (SINR) estimates, hybrid automaticrepeat-request (HARQ) state, or modulation and coding scheme (MCS)processes can be maintained to determine when antenna selection isrequired. The history can be collected by either the UE or the BS.

After a decision has been made, by either the UE or the BS, to performantenna training and selection, using e.g., a selection trigger signal401, the UE transmits the AS signals during the next TTI, by usingeither the AS signal 202 or the CQI signals as described above. Then,the BS can estimate the channels for both antennas, select an antenna210, and send the decision to use Tx2 205 back to the UE. In this case,the performance improves when the UE explicitly informs the BS about theform of the AS signal.

Training when Antenna Selection Occurs Between TTIs

We now describe the corresponding cases when antenna training andselection occurs between TTIs and not within a TTI as described above.Selecting between TTIs further simplifies the implementation complexityat the UE, with some extra delay in selecting the optimal set ofantennas.

Using Entire TTIs

FIGS. 5 and 6 shows how transmit antenna selection and training can beimplemented when the UE can only switch antennas between the TTIs. TheUE transmits the RBs 221 normally using the selected antenna. The RB istransmitted periodically using the unselected antenna Tx2. By using thechannel estimates from the previous TTIs, the BS can now select 210 theoptimal antenna for the UE, and feed back its decision Tx2 205 back tothe UE. This mechanism shows that a TTI can be used for antenna trainingand selection, as well as user data transmission.

FIG. 6 shows the same process for CQI signals 602. The TTI 601 that istransmitted with the unselected antenna includes data and the broadbandsounding RSs 602, and DM pilots, if present. As described above, usingthe TTI with the CQI signals also enables a joint resource blockreassignment and antenna selection. It should be noted, that theperiodicity of using the unselected antenna can vary from what isdescribed above.

However, the transmission with the unselected antenna Tx2 needs to bedone with a conservative lower rate MCS because the channel for Tx2 maynot be known at either the BS or UE. The RB that is transmitted with theunselected antenna includes data and pilot tones. While the initialtransmission with the unselected antenna requires a conservative choiceof the MCS, previous channel estimates coupled with the current channelestimate can be used to obtain a more reliable, and perhaps lessconservative MCS choice for sequent transmissions of RBs from theunselected antenna.

On-Demand Adaptive Training

FIGS. 7A and 7B show on-demand adaptive antenna selection in response tothe selection trigger 401. The UE transmits using the selected antenna(Tx1) until the performance of that antenna is less than a predeterminedthreshold, measured as described above. The UE sends the trigger signal401, and in the next TTI the UE initiates training by either sendingjust the AS pilot signal 701 with the unselected antenna as shown inFIG. 7A, or the entire RB 702 as shown in FIG. 7B.

In one embodiment of the invention, the UE reverts back to Tx1 for thesequent TTI, and for the select signal 205. In another embodiment, theUE continues to use antenna Tx2, unless the BS directs the UE to switchto another subset of antennas.

Multiple Antenna Subset Selection

In the examples below, we describe how antenna selection can beimplemented in a UE with two RF chains and four transmit antennas. Withtwo RF chains, the reference signals of two antennas are sentsimultaneously in a FDM or CDM manner as described above. The referencesignal sub-carriers for the different antennas are shown using twodifferent patterns.

FDM Pilots

As before, we describe antenna selection using the AS signals or thebroadband sounding reference signals. FIG. 8A shows periodically sendingthe AS signal 801 during every other TTI via a pair of unselectedantennas (Tx3 and Tx4), while FIG. 8B shows the AS signal 801 in everythird TTI.

CDM Pilots

FIGS. 9A-9B show antenna training and selection with periodic FDMbroadband sounding reference signals 901 and RB assignment, for everyother TTI and every third TTI, respectively using a pair of unselectedantennas, e.g., Tx3 and Tx4.

FIGS. 10A-10B show antenna set selection by using periodic CDM datamodulation signals. In this case, the two pilots 1001-1002 transmittedconcurrently are orthogonal to each other. Similar schemes are possiblewhen the UE switches between TTIs, and for on demand (adaptive) antennaselection.

Antenna Training for One RF Chain and Four Transmit Antennas

An embodiment for one RF chain and four antennas as shown in FIG. 11.The UE transmits the training information for four antennas, such thatonly one transmit antenna is active at any one time. We describe threeoptions, although other generalizations and combinations are alsopossible.

As shown in FIG. 11, the UE sends a data packet in the first TTI 1101from Tx1, and uses SB2 to send the AS signal 202 for Tx2. Then, the BScan determine which of the antennas Tx1 and Tx2 is better, and feeds itsdecision, e.g., use Tx2 1105, back to the UE. This feedback is receivedby the UE after the third TTI. In the meantime, the UE retransmits thesecond data packet in the second TTI 1102 from Tx1, and uses SB2 of theRB to send the AS signal for Tx3. Then, the transmitter switches to Tx2,as earlier indicated by the BS, and transmits the third data packet inTTI 1103 using Tx2. In the same TTI, the UE uses the SB2 to send theantenna selection signals for the last remaining antenna Tx4. Then, theBS determines, for example, that Tx3 is the optimal of all fourantennas, and indicates to UE to transmit using Tx3. The UE thentransmits the data packets 1104 using Tx3 1103. A similar mechanism canbe described for the 1 ms TTI with 14 LBs in which 2 LBs carry referencesignals.

Note that the BS updates its selection decision and feeds the decisionback while estimating the channels for the different antennas. In oneembodiment, the BS only feed back its final decision, withoutincremental selection updates. In this case, the feedback to use Tx2 isabsent, and the UE transmits the third TTI using Tx1.

FIG. 12 shows another option to speed up the selection process. Thisoption uses a combination of selecting within a TTI and selectingbetween TTIs. The UE transmits a data packet using antenna Tx1 in thefirst TTI 1201, and uses SB2 to send the AS signal 1202 from antennaTx2. Then, the UE switches to antenna Tx3 to transmit the data packet inthe second TTI 1203, and sends the AS signal 1204 for antenna Tx4 inSB2.

Then, the BS can determine and compare the channel estimates from allthe four transmit antennas and feeds back its selection decision to theUE, e.g., Tx3 1205. The UE continues to transmit data packets fromantenna Tx1, while waiting for the selection decision, and switches toantenna Tx3 thereafter.

Alternatively as shown in FIG. 13, the BS send an incremental updatewhen the BS can estimate only a set of the channels of the multipleavailable antennas. The BS compares the channel estimates for antennasTx1 and Tx2 after the first TTI, and sends the selection decision 1301back to the UE. For example, the BS selects Tx2 1301. This decision isreceived by the UE after the second TTI. In the second TTI, the UE, asbefore, uses antenna Tx3 to transmit its data packet and antenna Tx4 forthe AS signal. However, in the third TTI, after receiving the BSsselection decision, the UE switches to Tx2 to transmit the data packet.As before, the BS can compare all the four antennas after the secondTTI, and send it selection decision, e.g., Tx3 1302, back to the UE. TheUE switches to Tx3 after the third TTI.

Using AS Packets

Stand Alone AS Packets

In addition to the embodiments described above, the antenna selectionprocess can also use an antenna selection (AS) packet 1400 as shown inFIG. 14. The AS packet embeds antenna selection control (ASC)information 1401 in, e.g., the first long block (LB1) and the DM pilot(P) signal 1402 in the first short block SB1 in case of a slot with 2SBs, or in LB4 in case of a TTI with 14 LBs. This process is verysuitable for traffic in bursts, as selection can be done just before atransmission burst. The ASC information 1401 can indicate which subsetsof antennas are being used by the UE to transmit the signals. Thus, theBS can directly associate its channel estimate with a specific antenna.In addition, the ASC information can also indicate an antenna selectionrequest by the UE, and that pilot tone in the second short block SB2should be used for training by the BS.

As shown in FIG. 14, the BS does not need to receive the pilot tonesbefore selecting an antenna. The BS can select immediately afterreceiving the first two OFDM symbols of the uplink TTI. This involvesthe following steps and timing delays. The BS receives the first andsecond OFDM symbols of the UL TTI from the UE and performs channelestimation and antenna selection with delay T₀. There is negligibleround trip propagation delay T₁ if the distance from the BS to the UE isless than 10 Km. The first long and short blocks of the DL TTI arereceived by the UE, and the UE then switches to the selected antennawith delay T₂.

Piggybacking AS Training

Alternatively as shown in FIG. 15, the UE can use control packets, suchas packets with ACK or NACK 1501 in LB1, for antenna selection. Thistype of packet is sent on the uplink after the UE receives a packet fromthe BS, even when the UE has no other uplink packets to transmit to theBS. To decrease the overhead of antenna selection, the ACS field 1401can be sent with some packets on otherwise unselected antennas.Therefore, this scheme requires no additional packets to be sent. The ASinformation can be piggybacked either in a periodic manner oradaptively, as described above. Furthermore, either the UE or the BS caninitiate this process.

Antenna Selection Methods

FIG. 16 shows an antenna selection method according to an embodiment ofthe invention. A first antenna is selected 1610 for transmitting signalsfrom the UE to the BS, e.g., the selected antenna Tx1 is the last usedantenna. It is assumed that the previous selection is known to the UEand the BS. The UE transmits 1620 a data packet to the BS via theselected antenna (Tx1) during a TTI. The data (or control information)are carried in long blocks of the data packet.

The UE also transmits 1630 an AS signal, as described above, using anunselected antenna, e.g., Tx2. The sending of the AS signal can beperiodic every k TTIs, or on demand. The AS signal is carried in a shortblock of the packet, or a subsequent data packet. The AS signal can be aFDM or CDM signal. As described herein, the AS signal can even be a lowoverhead signal. If the signal is a FDM signals, then low overheadimplies a smaller number of signal sub-carriers. For CDM signals, lowoverhead signals have reduced power.

In response to receiving the data packet and the AS signal, the BSestimates the channel and selects an antenna, and transmits theselection to the UE in step 1640. In the case of CDM signals, the BS canalso reassign the carrier frequencies of the resource blocks used by theUE. Then, after receiving 1650 the selection, and perhaps the RBassignment, the UE switches to the selected antenna for transmittingsequent packets.

If the antenna selection is on demand, the selection process can beinitiated by either the UE or BS based on the SINR, MCS, or HARQhistory.

FIG. 17 shows four ways (1701-1704) that antennas can be switched (SW)after they have been selected. The time required to switch antennas canbe measured in terms of nanoseconds, e.g., 10 to 100 nanosecondsdepending on the exact implementation. This is orders of magnitudeshorter than the length of the symbol, e.g., 10 ms.

Therefore, in one embodiment of the invention, the antennas are switchedsubstantially between the symbols. That is, the switching can take placeat the end of the block of the previous symbol, or at the beginning ofthe CP of the next symbol.

The four ways include: switching entirely within an LB/SB used to sendsignals, and use CP 1710 and LB or SB data part for switching—1701;switching using CP of LB/SB used to send pilot tones and CP of adjacentLB—1702; switching using CP of LB/SB used to send pilot tones and CP ofadjacent LB—1703; and switching using CP of adjacent LBs and not usingthe CP of the LB/SB used to send pilot tones—1704. Of these fourmethods, the first method, in which the LB/SB that contains pilot tonesfor transmission, is used for the switching times leads to the leastloss in performance because data LBs are not affected.

Reduced Antenna Sounding RS Overhead

It is also possible to reduce the overhead of antenna selection byreducing the frequency at which the AS RS is transmitted. In addition,the sounding reference signal can be transmitted even when no data aretransmitted. Furthermore, the base station can transmit its decisionabout which antenna the UE should use any time, including instances whenthe UE has not transmitted the sounding reference signal from theunselected antenna subset.

As shown in FIG. 18, the broadband sounding RS 1801 is transmittedalternatively from the two antennas. Note that the broadband SRS may betransmitted even when the UE does not transmit data. The base stationestimates the channels for the two antennas and performs resource blockand antenna scheduling. For example, the sounding RS period is two TTI1802, and the UE starts transmitting from TTI #1. Then, the sounding RSof TTI #1, 5, 9, . . . is transmitted from the first antenna and thesounding RS of TTI #3, 7 11, . . . is transmitted from the secondantenna.

One embodiment of the invention described below reduces the antennasounding overhead when the sounding RS is used as AS RS.

As shown in FIG. 19, the sounding RS 1901 is again transmittedperiodically. However, we now distinguish between the selected antennathat has most recently been selected by the eNodeB for data transmissionand the other unselected antenna. Now, only one in every k sounding RSsis transmitted from the unselected antenna and the rest RSs aretransmitted from the selected antenna, where k is greater than 1, forexample five, ten or fifteen. The optimal parameter value for k dependson the UE speed, the Doppler spread of the uplink wireless channel,scheduling constraints of the eNodeB, interference environment, etc. Asbefore, the eNodeB performs resource block assignment and decides whichantenna the UE should use for data transmission. The parameter k isknown to the eNodeB and the UE a priori.

The advantage of this scheme is that the eNodeB can estimate the channelfor the selected antenna more frequently. This is often the optimalantenna when the UE is moving slowly, or not at all. In both of theabove mechanisms, the eNodeB can know a priori when the unselectedantenna transmits the sounding RS.

As shown in FIG. 20, the sounding RS can be transmitted by the UE evenwhen the UE does not have data to transmit. A reduction in the ASsounding overhead when the DM RS is used for AS can be achieved in asimilar manner.

Simulation

In the following, we describe various adaptive antenna selectionschemes, and system-level simulation results with and without frequencydomain scheduling over different sounding RS usage parameters.

The simulation parameters are given in Table 1.

TABLE 1 System bandwidth 10 MHz TTI duration 1 ms Number of carriers perRB 12  Sounding RS location 1^(st) long block Number of LBs per TTI 14 Number of UEs per BS (cell) 25  Channel model 6-ray typical urban UEspeed 3 kmph Number of transmit (UE) antennas 2 Number of transmit RFchains 1 Number of receiver (eNodeB) antennas 2 Feedback delay 1 TTINormalized distance between transmit antennas 0.5 m Variance of angle ofdeparture 58° Spatial correlation of eNodeB (receive) antennasUncorrelated Number of contiguous RBs assigned to a UE 2 Schedulingalgorithm Frequency- domain 2. Fixed RB assignment

Simulation Results

Alternately Sounding the Two Antennas

We first consider the case in which the sounding RS is transmitted fromthe two antennas in an alternating manner as described above. Thetransmission interval of the sounding RS is set to either 2 ms or 10 ms.

The gains of adaptive antenna selection experienced by the user oversingle antenna selection are summarized in Table 2. The SNR gainexperienced by UE with adaptive antenna selection capability over a UEwith single transmit antenna for a two TTI sounding interval.

TABLE 2 2 TTI sounding interval With frequency- Without frequency- SNRCDF point domain scheduling domain scheduling  1% 3.2 dB 3.8 dB 10%   2dB 2.6 dB

The gains of adaptive antenna selection experienced by the userequipment over single antenna selection are summarized in Table 3. InTable 3, the SNR gain experienced by a UE with adaptive antennaselection capability over a UE with a single transmit antenna for theTTI sounding interval.

TABLE 3 10 TTI sounding interval With frequency- Without frequency- SNRCDF point domain scheduling domain scheduling  1% 2.4 dB 3.0 dB 10% 1.6dB 2.3 dB

In all cases, we see that the performance of the UE with adaptiveantenna switching capability, measured in terms of the SNR its linkexperiences, significantly improves.

Reduced Overhead Antenna Sounding

We now consider the case in which the sounding RS from the selectedantenna is transmitted less often from the unselected antenna. Thetransmission interval of the sounding RS is 2 ms. Sending the soundingRS from the unselected antenna less frequently, e.g., in only one out offive instances, has negligible loss in performance. Even adaptiveantenna selection based on the other more extreme cases, in which thesounding RS is sent from the unselected antenna in one out of ten, orone out of fifteen instances, still results in significant performancegains.

EFFECT OF THE INVENTION

The embodiments of the invention provide for antenna selection in theuplink from user equipment to a base station in a MIMO network, wherethe number of RF chains in the UE is less than the number of antennas.The invention also provides means for selecting antennas adaptively.

Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications may be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

1. A method for antenna selection (AS) in a wireless communicationnetwork, the network comprising user equipment (UE), wherein the UEcomprises a plurality of subsets of antennas including a first subset ofantennas and a second subset of antennas, and wherein the UE isconfigured to transmit a sounding reference signal (SRS) from a subsetof antennas at a time, comprising: transmitting a first SRS from thefirst subset of antennas; transmitting a second SRS from the secondsubset of antennas; receiving, in response to the transmitting the firstSRS and the second SRS, information identifying an optimal subset ofantennas from the first subset of antennas and the second subset ofantennas; and transmitting user data from the optimal subset ofantennas.
 2. The method of claim 1, further comprising: selecting theoptimal subset of antennas from the first and the second subset ofantennas based on the information.
 3. The method of claim 1, furthercomprising: estimating channels over multiple time-frequency resourceelements for the first subset of antennas and the second subset ofantennas based on the first SRS and the second SRS; and selecting theoptimal subset of antennas from the first and the second subset ofantennas based on the estimated channels.
 4. The method of claim 1,wherein the SRS is used by the network for assigning resource blocks tothe UE.
 5. The method of claim 1, wherein the UE includes only one radiofrequency (RF) chain.
 6. The method of claim 1, wherein the first SRSand the second SRS are transmitted in a time division multiplexedmanner.
 7. The method of claim 1, wherein the first SRS is transmittedin a first subframe and the second SRS are transmitted in a secondsubframe.
 8. The method of claim 1, wherein the first and the second SRSare transmitted using orthogonal frequency division multiplexing.
 9. Themethod of claim 1, in which the transmitting of the first and the secondSRS is periodic.
 10. The method of claim 1, in which the first and thesecond SRS are transmitted when demanded by the network.
 11. The methodof claim 1, wherein the SRS is included in every subframe transmitted bythe UE.
 12. The method of claim 1, wherein the SRS is included in everymultiple subframes transmitted by the UE.
 13. The method of claim 1,wherein the subset of antennas that most recently transmitted the userdata is a selected antenna subset of the set of available antennas andother antennas of the set of available antennas is an unselected antennasubset, and further comprising: transmitting the SRS less frequentlyfrom the unselected antenna subset than from the selected antennasubset.
 14. The method of claim 1, wherein the subset of antennas thatmost recently transmitted the user data is a selected antenna subset ofthe set of available antennas and other antennas of the set of availableantennas is an unselected antenna subset, and further comprising:transmitting the SRS more frequently from the unselected antenna subsetthan from the selected antenna subset.
 15. The method of claim 1,wherein the information is based only on the first and the second SRS.16. A user equipment (UE), the UE is configured to transmit a soundingreference signal (SRS) from a subset of antennas at a time, comprising:a first subset of antennas configured to transmit a first SRS; a secondsubset of antennas configured to transmit a second SRS; a receivingmodule configured to receive, in response to the transmitting the firstSRS and the second SRS, information identifying an optimal subset ofantennas from the first subset of antennas and the second subset ofantennas; and transmitter configured to transmit user data from theoptimal subset of antennas.
 17. The UE of claim 16, further comprising:selecting module configured to select the optimal subset of antennasfrom the first subset of antennas and the second subset of antennasbased on the information.
 18. The UE of claim 16, further comprising: ameans for switching between the first subset of antennas and the secondsubset of antennas based on the information related to the optimalsubset of antennas.
 19. A wireless communication network, the networkcomprising user equipment (UE), wherein the UE comprises a plurality ofsubsets of antennas including a first subset of antennas and a secondsubset of antennas such that UE is adapted for antenna selection (AS),and wherein the UE is configured to transmit a sounding reference signal(SRS) from a subset of antennas at a time, comprising: means fortransmitting a first SRS from the first subset of antennas; means fortransmitting a second SRS from the second subset of antennas; means forreceiving, in response to the transmitting the first SRS and the secondSRS, information identifying an optimal subset of antennas from thefirst subset of antennas and the second subset of antennas; and meansfor transmitting user data from the optimal subset of antennas.
 20. Thenetwork of claim 19, further comprising: means for selecting the optimalsubset of antennas from the first and the second subset of antennasbased on the information.